1. Field of Invention
This invention relates to object inspection techniques, and more particularly to optical apparatus and method for detecting edges of parts or objects with high precision and repeatability.
One basis for measuring the dimensions of a part or object is finding suitable edges or surfaces accurately and repeatably. Many parts are contact-sensitive or moving too fast in automated handling for contact methods. For low-cost manual operations, this is usually accomplished with a microscope having calibrated cross-hairs in the eyepiece. The operator judges the location of the edges of the part and then aligns the cross-hairs with such edges in order to read the dimension. This labor-intensive process is subjective and susceptible to operator fatigue and variances.
Certain known automated measuring techniques use a video camera in conjunction with the microscope. An electronic "hairline" along with computer image enhancement assists the operator in the measurement process but usually cannot find the edges without operator assistance. Systems of this type having edge-finding capabilities typically cost over $50,000, are fairly slow, and usually are limited to use in the metrology laboratory rather than on the production floor where such capability is needed.
The current trend is toward integrating the design, manufacture and inspection of parts under fully automated control using Computer Aided Engineering (CAE), Computer Aided Design/Drafting (CADD), Computer Aided Manufacturing (CAM), and Computer Aided Inspection (CAI). Much progress has been made in all but the last area. Coordinate Measuring Machines (CMMs) are available which can automatically measure parts in an inspection laboratory environment, but because of their size and cost, such machines have not been effectively integrated into the manufacturing process. Measurements of this kind are almost exclusively done in the metrology laboratory in which the environment can be controlled, but which is usually remotely located with respect to the fabrication site.
Automated Optical Inspection (AOI) is a desirable "in-process" method of inspection in automated manufacturing that can remove the subjectivity of set-up and inspections during fabrication while allowing corrections of measurement parameters to be made immediately, resulting in more precise parts at higher yields in less time. Because AOI is an optical, non-contact technique, it removes many of the speed and part accessibility limits usually encountered in conventional measuring techniques and provides precision and accuracy previously unattainable, for example, with contact inspection techniques. Various optical, non-contacting measuring and inspection schemes are described in the literature (see, for example, U.S. Pat. Nos. 4,422,763; 4,384,195; 4,583,854; 3,879,131; 4,201,476; 3,856,412; 4,272,190; 4,597,668; 4,624,563; and 4,427,296).
Optical measurement techniques are ideally suited for high-speed, precision fabrication of metal parts, electronic assemblies, semiconductor components, and for numerous other manufacturing tasks such as alignment, inspection, tracking, and the like, within diverse segments of commerce. For example, measurement microscopes as described in the literature, are commonly used in the inspection or alignment of small and delicate parts. They are often equipped with "encoders" which precisely display the position of a part with respect to the cross-hairs seen through the eyepiece. A camera port is typically used to mount a video camera which aids the operator in the inspection process. Because these camera ports are well-standardized, any instrument with such a feature can support a variety of accessories, including an edge-finding accessory of the type later described herein. Also, in the electronics industry there are numerous measurement and alignment techniques employed at all stages of circuit chip and circuit board fabrication. Many of these techniques use video-based inspection equipment in which the accuracy is limited to the field of view of the microscope and camera combination. In addition, robotic arms used in production applications commonly reference an edge or measure where a sample edge is located. In one such application, a robotic arm may be controlled to lay down a bead of weld along a seam and in another application a robotic arm may be coordinated with a measuring machine to inspect parts for dimensional accuracy. These and other such applications usually require the ability to detect an edge from light reflected from the object rather than from light passed through the object.